Microsoft today opened registration for its limited-time Windows upgrade offer, which provides a discount to buyers of new Windows 7-based PCs who want to upgrade in the future to the Windows 8 Pro edition.

The offer just applies to those moving to Windows 8 Pro on x86/x64 machines. Windows RT, the operating system that runs on ARM-based hardware, will only be sold preinstalled on machines; it isn't possible to download it or buy boxed copies.

The offer to upgrade to Windows 8 Pro for $14.99 is good only for owners of Windows 7 PCs that were purchased between June 2, 2012 and Jan. 31, 2013, according to Microsoft's announcement. An announcement made last month seemed to suggest that the offer was available with Windows XP and Vista machines too, but that's not exactly correct. The main problem is that people can't buy new PCs running those operating systems, and the offer is only extended to new PC buyers. However, the upgrade offer is still available to those people who bought a new Windows 7 PC who then downgraded that OS to Windows XP or Vista. Such downgrades do not affect the upgrade offer, or even the process of upgrading to Windows 8, according to Microsoft's FAQ.

Offer Limitations
Those who bought retail outlet-sold boxed copies of Windows 7 that weren't installed beforehand on PCs are out of luck, according to the FAQ. And forget about the offer as well if you bought a refurbished PC with Windows 7 preinstalled. It doesn't qualify, since it isn't new.

Microsoft is also stipulating that the offer is just for consumers and not for businesses. An individual can take advantage of the offer on five upgrades, but no more.

The offer is a multistep process. The new Windows 7 PC has to be purchased within the dates of the offer. Next, those wanting to upgrade to Windows 8 have to register, and that's now open via this page. Lastly, the upgrade itself is only available when Windows 8 is released as a final product on October 26, so those signing up can't upgrade right away. Finally, the offer has a time-out clause, even for those who signed up -- that is, the Windows 8 download has to take place before Feb. 28, 2013 or the deal is null.

The upgrade happens over an Internet connection by means of a "Windows upgrade assistant" service. It can take "30 to 90 minutes" to download Windows 8, which is a 2 GB file. Of course it's not possible to upgrade a 32-bit Windows 7 PC with a 64-bit edition of Windows 8. Microsoft will provide Windows 8 on installation media for an additional charge, or upgraders can burn their own installation disks on a DVD disk or a USB drive.

Potential Incompatibilities
The Windows upgrade assistant will check if a PC is capable of running Windows 8. It provides information to the user during the upgrade process. Microsoft's FAQ says that "this might include information about software that requires updates to properly function on Windows 8, devices which require updated drivers, or software and devices that may not work after upgrading." This statement might be the first indication from Microsoft that users could encounter such potential incompatibilities. Up to this point, Microsoft executives have made very general statements assuring that Windows 7 logo-qualified PCs should be capable of running Windows 8.

Some people installing Windows 8 have noted that the ability to run Hyper-V on Windows 8 can be a question. That ability depends on the type of Intel processor that's in the Windows 7 machine prior to the Windows 8 upgrade. The processor has to have the right Intel virtualization technology (VT) to work. Hyper-V virtualization on AMD machines likely will work.

Upgraders face a few other considerations too. Microsoft offers a nuanced explanation about how well touchscreen-enabled Windows 7 PCs will work once upgraded to Windows 8. The FAQ states that the touch-screen experience will be "at least as good as it was on Windows 7." It appears that users need to know how many touch points their touchscreen supports. The FAQ explains that "touchscreens supporting fewer than five simultaneous touch points cannot enable certain Windows 8 features and applications."

The screen has to be the right size, too. To run apps from the Windows Store (sometimes called "Metro-style" apps or maybe "modern" apps) and even the built-in Microsoft Windows 8 apps, users need a screen resolution of "at least 1024 x 768." If the screen doesn't have a 1366 x 768 dimension, users won't be able to snap apps side by side in Windows 8.

Microsoft had announced earlier in May that it wasn't providing DVD playback capability built into Windows 8 by default, but that it would be available as upgrade option for Windows 8 Pro users for an unannounced price. It will also be possible to use third-party media players on Windows 8. So, if the Windows upgrade assistant determines that the existing media player on Windows 7 is incompatible with Windows 8, upgraders would need to get and install "third-party playback software" or get the Windows 8 Media Center Pack after upgrading, when that becomes available.

For those with machines not qualifying for this offer, Microsoft will still offer a discount upgrade on October 26, when Windows 8 is released. At that time, it will be possible to upgrade from Windows 7 to Windows 8 for $39.99, according to Microsoft's announcement.